Dynamically tunable, single pixel full-color plasmonic display, method and applications

ABSTRACT

Dynamic, color-changing surfaces have many applications including but not limited to displays, wearables, and active camouflage. Plasmonic nanostructures can fill this role with the advantages of ultra-small pixels, high reflectivity, and post-fabrication tuning through control of the surrounding media. However, while post-fabrication tuning have yet to cover a full red-green-blue (RGB) color basis set with a single nanostructure of singular dimensions, the present invention contemplates a novel LC-based apparatus and methods that enable such tuning and demonstrates a liquid crystal-plasmonic system that covers the full red/green/blue (RGB) color basis set, as a function only of voltage. This is accomplished through a surface morphology-induced, polarization dependent, plasmonic resonance and a combination of bulk and surface liquid crystal effects that manifest at different voltages. The resulting LC-plasmonic system provides an unprecedented color range for a single plasmonic nanostructure, eliminating the need for the three spatially static sub-pixels of current displays. The system&#39;s compatibility with existing LCD technology is possible by integrating it with a commercially available thin-film-transistor (TFT) array. The imprinted surface readily interfaces with computers to display images as well as video.

CROSS-REFERENCE TO RELATED APPLICATION

The present application relates and claims priority to U.S. ProvisionalPatent Application Ser. No. 62/481,178, filed Apr. 4, 2017, the entiretyof which is hereby incorporated by reference.

STATEMENT OF GOVERNMENT INTEREST

Funding for the invention was provided by the National ScienceFoundation under award NSF ECCS-1509729. The U.S. government has certainrights in the invention.

BACKGROUND

Aspects and embodiments of the invention pertain to nanostructured,plasmonic-enabled, liquid crystal (LC) display apparatus and methods,and more particulary to a voltage-controlled liquid crystal-plasmonicdisplay apparatus, methods, and applications, that cover the full RGBcolor basis set with a single nanostructure of singular dimensions.

Structural color arising from plasmonic nanomaterials and surfaces hasrecieved ever increasing attention. The drive to commercialize thesesystems has led to significant improvements in color quality, angleindependence, brightness, and post-fabrication tunability. However,while most of these advances struggle to replace present commerciallyavailable technologies, the ability to change color, post-fabrication,is a distinct advantage of plasmonic systems, which may allow them toadvantageously fill niche applications. For example, traditionaltransmissive and reflective displays typically have three sub-pixelregions with static red, green and blue color filters. These sub-pixelscontrol the amount of each basis color transmitted or absorbed to createarbitrary colors through a process called color mixing.

A display built from a dynamic color-changing surface can eliminate theneed for individual sub-pixels, increasing resolution by 3× withoutreducing pixel dimensions. However, previous reports of post-fabricationplasmonic tuning have yet to span an entire color basis set (RGB or CYM)with a single nanostructure.

In Applicant's previous work (described in PCT/US2015/056373), ananostructured, plasmonic-enabled, color generation apparatus isdisclosed that could only span two of the three RGB values for a singlenanostructure. In order to cover the entire color set, multiplenanostructures were required.

SUMMARY OF THE INVENTION

According to the embodied invention, we demonstrate a reflectivecolor-changing surface capable of producing the full RGB color basisset, all as a function of voltage only, and based on a singlenanostructure. This is achieved through the onset of surfaceroughness-induced polarization dependence and a combination ofinterfacial and bulk LC effects. Each of these phenomenon dictates thecolor of the surface within different voltage regimes: bulk LCreorientation leading to polarization rotation in the low voltageregime, and surface LC reorientation leading to plasmonic resonanceshifting at higher voltages. This hybrid LC-plasmonic tuning mechanismis modeled through a combination of finite element (FEM), Jonescalculus, and finite difference time domain (FDTD) simulationtechniques. Lastly, the scalability and compatability of this systemwith existing LCD technology through integration with a TFT array isdemonstrated. The resultant device is then interfaced with a computer todisplay arbitrary images and video. This work demonstrates the potentialof hybrid LC-plasmonic systems for single pixel, full-color, highresolution displays and color changing surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an schematic representation of an illustrative embodiment of aliquid crystal-plasmonic apparatus;

FIG. 2(a) is a reflection spectra for incident light polarizedperpendicular (90°) to a liquid crystal long-axis;

FIG. 2(b) is a reflection spectra for incident light polarized parallel(0°) to a liquid crystal orientation;

FIG. 2(c) is SEM images and a corresponding histogram of 30 nm “smooth”aluminum film on an imprinted polymer;

FIG. 2(d) is SEM images and a corresponding histogram of 30 nm “rough”aluminum film on an imprinted polymer;

FIG. 2(e) is a reflection spectra compared to a simulated spectra of the“smooth” aluminum film for both the parallel (0°) and perpendicular(90°) incident polarizations;

FIG. 2(f) is a reflection spectra compared to a simulated spectra of the“rough” aluminum film for both the parallel (0°) and perpendicular (90°)incident polarizations;

FIG. 3(a) is a schematic representation of the liquid crystal and lightpolarization through a cell at three selected voltages;

FIG. 3(b) is a schematic representation of an experimental setup toverify liquid crystal orientation throughout a cell;

FIG. 3(c) is a graph depicting the degree of polarization rotationimparted on the light as a function of voltage;

FIG. 3(d) is a reflection spectra of two orthogonal off states of theplasmonic surface when excited with parallel and perpendicular lightwith respect to a liquid crystal director;

FIG. 3(e) is a reflection spectra of a surface in a low applied fieldregime and a simulated spectra obtained by a superposition of twoplasmonic modes;

FIG. 4(a) is a measured and simulated reflection spectra of a surface asa function of applied field and input polarization;

FIG. 4(b) is camera images of a large area sample through a polarizingfilm for selected polarizations and voltages;

FIG. 4(c) is a CIE chromaticity diagram and microscrope images of asurface as a function of voltage when the polarizer, liquid crystaldirector, and analyzer are parallel;

FIG. 5(a) is a schematic representation of the polarization of incidentlight parallel to the liquid crystal director where the angles ofincidence are φ and θ;

FIG. 5(b) is a graph of experimental measurements when the axis ofrotation is parallel (φ, TE, s-polarized) and perpendicular (θ, TM,polarized) to the incident light;

FIG. 6(a) is a microscope image of the plasmonic surface integrated witha thin-film-transistor (TFT) array (10×);

FIG. 6(b) is a depiction of the surface color changes after applying avoltage to every 3^(rd) and 4^(th) row of the TFT;

FIG. 6(c) is a pair of full images displayed from the device (4×);

FIG. 6(d) is a passively addressed device with UV lithographicallypatterned “UCF” as a function of applied, field, polarizer, and analyzeralignment and LC photoalignment director;

FIG. 7 is a series of SEM images for “rough” and a“smooth” aluminum thinfilm at various steps of a watershed method;

FIG. 8(a) is a depiction of the off-state behavior of the surfaceresults in a blue color when the electric field of incident light isparallel;

FIG. 8(b) is a depiction of the off-state behavior of the surfaceresults in a red color when the electric field of incident light isperpendicular;

FIG. 8(c) is a depiction of the off-state behavior of the surfaceresulting in a purple color when unpolarized;

FIG. 9(a) is a schematic representation of a unit cell of a simulatednano-well array;

FIG. 9(b) is a series of graphs showing the resulting φ and θ from FIG.9(a) as a function of the cell position and voltage;

FIG. 10(a) is a graph showing the wavelength and voltage dependentweighting factors for φ;

FIG. 10b ) is a graph showing the wavelength and voltage dependentweighting factors for β;

FIG. 11(a) is a graph showing response time measurements for 2.6 V (1kHz AC);

FIG. 11(b) is a graph showing response time measurements for 5.4 V (1kHz AC);

FIG. 12(a) is an optical micrograph at 0 V/μm of a singular Afghan Girlimage (National Geographic Society) as a function of applied electricfield;

FIG. 12(b) is another optical micrograph at 1.25 V/μm of a singularAfghan Girl image (National Geographic Society) as a function of appliedelectric field;

FIG. 12(c) is an optical micrograph at 2.5 V/μm of a singular AfghanGirl image (National Geographic Society) as a function of appliedelectric field;

FIG. 12(d) is an optical micrograph at 10 V/μm of a singular Afghan Girlimage (National Geographic Society) as a function of applied electricfield;

FIG. 12(e) is an optical micrograph at 10 V/μm in FIG. 12(d) with 10×objective;

FIG. 12(f) is a SEM image of the sample before fabrication into a liquidcrystal cell;

FIG. 12(g) is another SEM image of the sample before fabrication into aliquid crystal cell; and

FIG. 12(h) is yet another SEM image of the sample before fabricationinto a liquid crystal cell.

DETAILED DESCRIPTION Liquid Crystal-Plasmonic Device

A schematic of the liquid crystal-plasmonic apparatus is shown inFIG. 1. At the top of the device, unpolarized ambient light passesthrough a linear polarizer, glass superstrate, indium tin oxide (ITO),and a rubbed polyimide film. The ITO serves as a transparent electrodefor applying electric fields across the liquid crystal, and the rubbedpolyimide aligns the LC parallel to the axis it is rubbed (homogeneousalignment). The polarized light continues through a high birefringenceLC layer (LCM1107, LC Matter Corp.) and excites grating coupled surfaceplasmons (GSCP) on the nanostructured aluminum surface. The LCorientation near the plasmonic surface determines the effectiverefractive index of the GCSP modes and, therefore, the resonantwavelength. Light that is not absorbed by the nanostructure is reflectedback from the device resulting in a perceived color. The plasmonicsurface constitutes the second half of the LC cell and is fabricatedthrough the nanoimprint lithography of a 300 nm period nanowell arraywith an impression depth of 80 nm, which is followed by a 30 nm-thickaluminum electron beam evaporation. By changing deposition conditionsand substrate temperatures the surface quality and oxide content of thefilm can be greatly varied. This creates a relatively rough aluminumsurface (˜35 nm rms), which induces a polarization dependence on theGSCP resonance in the presence of an anisotropic media, whereas it wasshown in Applicant's previous work (id.) that a smooth surface (˜10 nmrms) results in near polarization independence. This results in thedevice having two orthogonal color states in the “voltage off” state(V_(off)), depending on the polarization of exciting light eitherparallel (blue) or perpendicular (red) to the LC director near thenanostructure. Also from this previous work, we found the periodicnanowell structure imparts a weak diagonal alignment on the LC withrespect to the structure's grating vectors. We use this to create a 45°twisted nematic cell by aligning the top rubbed polyimide layer with oneof the grating vectors of the nanostructure. By applying a voltageacross the plasmonic film and the top ITO, the orientation of the LCthroughout the cell is controlled. At intermediate voltages, the bulk LCreorients and retards the incident light resulting in a partialexcitation of the surface's two orthogonal “off-state” modes. At aspecific voltage (V_(th)), however, the polarization is effectivelyrotated to excite the opposite plasmonic mode, marking a transition incolor from red to blue or blue to red. This birefringent effect iswavelength dependent and can add an oscillatory absorption to thereflection spectra as light is partially absorbed on the second passthrough the polarizer. However, we will show below that when thepolarizer is aligned in parallel with the LC director, the color of thesurface is independent of this effect. At higher voltages the bulk LCsaturates vertically while the LC near the aluminum surface begins toreorient, which increases the effective refractive index experienced bythe plasmonic nanostructure. This results in a continuously tunable redshift of the GCSP resonance, eventually saturating when the LC near thesurface is also completely reoriented by the electric field. In thisstate (V_(s)), the surface turns green and loses its polarizationdependence.

Polarization Dependent Color

Surface plasmon resonances depend greatly on the local morphology of themetallic surface and behavior can vary drastically with surfaceroughness and oxide content. An increase in the roughness of a metalfilm causes a red-shift in the resonant plasmon wavelengths and iscommonly attributed to an increase in scattering. Having a distributionof roughness and grain size within a film will then have a broadeningeffect on the resonance. While this is normally considered detrimentalfor plasmonic applications like biomolecular sensing and SERSenhancement, this effect can be used advantageously for plasmonicstructural color as structures that absorb broad wavelengths of lightare able to produce colors not possible from those with only narrowabsorption resonances. To demonstrate the impact of surface roughness onthe color reflected from the nanostructure, we perform FDTD simulationsof the periodic nanowell array as a function of rms roughness of thealuminum film and use the spectra to predict a reflected color using theCIE color matching functions. Here, we approximate the liquid crystalregion as a perfect anisotropic crystal with the slow axis parallel tothe surface and at 45° with respect to a periodicity vector of thenanowell array (homogeneous LC alignment). This isolates the fundamentalmodes of the plasmonic film from the bulk retardation/polarizationrotation effects of the LC. The results for incident light polarizedperpendicular to the liquid crystal long-axis (90°) is shown in FIG.2(a), while incident light polarized parallel (0°) to the liquid crystalorientation is shown in FIG. 2(b). Line colors are determined by the CIEcolor matching functions and are cascaded to show the influence ofsurface roughness on resonance location (solid black lines) andfull-width-half-maximum (dotted black lines). At low values of surfaceroughness the plasmonic resonance shifts less than 20 nm upon incidentpolarization rotation, resulting in a minute color change. However, asthe roughness of the aluminum increases the parallel resonance redshifts more than the perpendicular case causing a greater color shiftbetween the two polarization states as can be seen from FIG. 2a -b. Weexplain this phenomenon with the following argument: as plasmonicresonances depend on the surface normal component of the surroundingrefractive index, a rough film will have local regions where the surfacenorm has an increased x or y component compared to the global norm. Thisgives the in-plane components of the surrounding media a greatercontribution to the effective refractive index of the GSCP resonance. Inthe current case, where the surrounding media is a uniaxial crystaloriented parallel to the aluminum surface, this creates a surfaceroughness induced polarization dependence of the plasmonic resonance.This effect has been experimentally demonstrated with two surfacemorphologies labelled as A and B that are obtained by controllingdeposition parameters and substrate temperatures, detailed descriptionsof which can be found in the Methods section herein below. Thecorresponding top and cross-sectional scanning electron microscopeimages of the 30 nm aluminum films on the imprinted polymer are shown inFIG. 2(c-d). A watershed-based image segmentation method is used to findthe distribution of grain diameters from the top-view SEM images (FIG.2(c-d)). This method is described in detail in FIG. 7. In FIG. 7, theinput SEM images for the “rough” and “smooth” aluminum thin film areshown at various steps in the watershed-based image segmentation method.The method results in a list of regions with the number of pixelscontained therein. The area of each region is then converted into anapproximate grain diameter. Thereafter, the regions are mapped back ontothe image in the form of white lines to confirm the methodsfunctionality. Histograms of the two surfaces show a mean grain diameterof 12 nm for the “A” surface (FIGS. 2(c)) and 33 nm for the “B” aluminumsurface (FIG. 2(d)). The surfaces are then made into LC cells andinfiltrated with LCM1107. FIG. 2(e-f) shows the resulting reflectionspectra of the respective surfaces (solid lines) compared to simulatedspectra (dotted lines) for incident polarizations parallel (0°) andperpendicular (90°) to the top rubbed polyimide alignment layer. Outputlight of all polarizations is collected to ensure the spectra originatepurely from the plasmonic surface. Line colors of the spectra aregenerated by the CIE color matching method while the microscope imageinsets show the experimentally achieved colors. The histograms of therespective aluminum surfaces are used in a weighted average of the FDTDspectra of FIG. 2(a-b) and result in a close matching to theexperimental spectra. The relief depth of the structure in FDTDsimulations has also been varied between 80-90 nm to provide a bettermatching to experimental spectra and account for small variances in theimprinting process. The relatively smooth aluminum film, A, gives asmall change in color upon polarization rotation, from an orange to areddish-orange, associated with a 20 nm shift in GCSP resonance. On thecontrary, the aluminum surface, B, with higher surface roughness gives amore drastic color change upon polarization rotation, from blue to red(FIG. 2(f)) and is the result of a 60 nm shift in resonant wavelength.Hence, by controlling the surface morphology via deposition conditions,the degree of polarization dependence of the resultant color can becontrolled. Exciting the surface with polarization angles between 0° and90° or unpolarized light results in a superposition of these two spectra(FIG. 8(a-b)). An example of this is that the surface B when excitedwith unpolarized light results in a purple color (FIG. 8(c)).

Liquid Crystal Phase Retardation

To achieve all three basis colors (RGB) as a function only of voltage acombination of bulk and surface liquid crystal effects are utilized. Theperiodic nanowell structure imparts a weak diagonal alignment on the LCwith respect to the structure's grating vectors, which is used to createa 45° twisted nematic cell by aligning the top rubbed polyimide layerwith one of the grating vectors of the nanostructure. A cell gap of 8.5μm ensures the alignment of the top polyimide does not overpower that ofthe nanostructure to result in a homogeneous cell. These boundaryconditions, along with the LC material parameters are used in FEMsimulations (TechWiz LCD 3D, Sanayi) to find the LC director throughoutthe cell as a function of voltage. A Jones matrix approach is then usedto find the polarization of light as it propagates through the cell.This is done by approximating the LC cell as a stack of N number ofphase plates with a continuous variation in retardation. The LC directorat that cell location determines the anisotropic refractive index ofeach layer. A detailed description of the process is provided hereinbelow. The resulting LC directors and polarization ellipse throughoutthe cell for three selected voltages are represented in FIG. 3(a). Whilethe FEM and Jones matrix calculations are found to converge with N=100,the figures in FIG. 3(a) depict a gridded subsampling for graphicalpurposes. The top row shows incident polarized light parallel to the topLC director as it propagates down towards the nanostructure, while thebottom row depicts the light after reflection and as it propagates outof the cell. At V=0 (A), the LC creates a 45° twisted nematic cell wherethe propagating light maintains its linear polarization and is rotatedby the LC director in what's known as the Maugin (or Waveguiding)regime. This is given by the following condition:

${\varphi {\operatorname{<<}\; \frac{2\pi}{\pi}}\Delta \; {nd}},$

where ϕ is the twist angle of the cell, An is the birefringence of theLC, λ is the wavelength of light, and d is the thickness of the LC cell.Here we can see that light exits the cell with the same polarization asinput and is, therefore, unaffected by a second pass through thepolarizer. As voltage is increased, the Fredericks transition marks theinitial tilt of the LC from their voltage-off state and is followed by acontinuous change in tilt along the applied electric field. This tiltingreduces the effective Δn of the cell, which eventually breaks the Maugincondition and begins changing the ellipticity of the light, as can beseen in (B) and (C) of FIG. 3(a). To verify this LC mode, we use theexperimental setup illustrated in FIG. 3(b) and compare with simulationsfrom the combined FEM-Jones approach. The experiment consists of lightfrom a He—Ne laser that passes through a polarizing beam splitter (PBS)and is incident upon the plasmonic-LC device. In this case, thepolarization of incident light is parallel to the rubbing of the topalignment layer. Light is reflected from the device and back into thePBS, which reflects the orthogonal polarization of light to a siliconphotodiode. A voltage is then adiabatically applied to the sample at arate of 0.01 Vs⁻¹, well below that of the cell's transient opticaleffects. The results of the experiment are shown in FIG. 3(c) andrepresent the degree of polarization rotation imparted on the light as afunction of voltage. Jones matrix simulations of a 45° twisted nematiccell with a parallel input polarizer and perpendicular output analyzer(both with respect to the top LC director) match well with theexperimental curve. A close to zero reflection indicates that the lightis leaving the cell with the same polarization as it entered. This canonly occur if the light reflects off the plasmonic surface in a linearpolarization state, as can be seen in the selected voltage cases (A) and(B). On the contrary, peaks in the curve indicate voltages at whichlight leaves the cell at a perpendicular polarization than as itentered, which occurs due to the change in hand of reflected circularpolarized light (C). While this verifies the bulk LC mechanics of thecell, to understand this effect's impact on the color of reflected lightwe look at the polarization of light as it excites the plasmonicsurface. The two orthogonal modes of the surface-B are shown in FIG.3(d). These spectra are obtained at 0 V and when the polarization ofincident light is parallel (0°) and perpendicular (90°) to the toppolyimide alignment layer, respectively. Since the cell is in the Mauginregime at this state, the incident light remains either parallel orperpendicular to the LC director as it excites the plasmonic surface.Within a low-voltage regime, where only the bulk LC changes orientationand the LC near the surface remains constant, the spectra is a linearcombination of the two orthogonal basis spectra with a wavelengthdependent weighting, α and β. These weighting terms are given by theprojection of the electric field exciting the nanostructure on the axesparallel and perpendicular to the LC director, respectively. FIG. 3(e)shows experimentally obtained reflection spectra of the surface with avoltage of 3.5 V (V_(th)) where, for a given incident polarization, thespectra flips to the orthogonal state's color. Using the Jones Matrixmethod to find α and β, the resulting spectra closely match withexperiment. Detailed steps for obtaining α and β, as well as their exactvalues for this case are provided herein below.

As voltage is increased, the LC near the nanostructure and within theplasmonic fields of the surface begins to tilt. This increases theeffective refractive index of the GCSP modes and continuously red-shiftstheir resonant wavelengths. Once this occurs, the two state approachused above is no longer valid, but also not needed to predict thedevice's end-state reflection spectra. At saturation voltage (V_(s)),the LC near the surface becomes asymptotically vertical, which resultsin the loss of the nanostructure's polarization dependence, as well asthe bulk LC's birefringent properties. This is demonstrated in FIG. 4(a)where the voltage and polarization dependent reflection spectra of thedevice is shown. Again, line colors are generated by overlap integrationbetween the measured spectra and the CIE color matching functions andinsets show optical microscope images of the surface. The two top rowsof spectra are of the surface when excited with polarized light paralleland perpendicular to the LC alignment director, respectively, and lightof all polarizations are collected. While this confirms that the coloris generated purely by the plasmonic surface, light from a practicaldevice, in which the polarizer is laminated to the top, would passthrough the polarizer a second time. The bottom-most row of spectra inFIG. 4(a) shows the influence of this second pass on the color of thesurface as an analyzer is added in parallel with the polarizer. In theoff-state, the spectra are invariant to the addition of an analyzer asthe LC is in the waveguiding regime as described in FIG. 3(a). For thisstate, the experimental spectra match closely with thehistogram-weighted FDTD spectra. At an applied voltage of 3.5 V, thereflected color flips due to the polarization rotation of incident lightas it passes through the LC cell. The simulated spectra for this appliedfield is obtained by using the wavelength dependent weighting factors, αand β, on the FDTD results at V=0. The second pass through the polarizeris simulated by adding an analyzer to the Jones calculus method. Here,there is a slight change in spectra with the addition of the analyzer,however, the changes do not greatly impact the reflected spectra andcolor. At saturation voltage (LC on state), the first order plasmonicresonance shifts to 600 nm and a second order resonance moves to 500 nm.This results in a green color reflected from the surface, which can beconfirmed by FDTD with an anisotropic media where n_(z)=n_(e) andn_(x)=n_(y)=n_(o) of the LC. Here, we also see that the effect of theanalyzer is minimal, which is consistent with the LC being nearlyvertical throughout the cell. To demonstrate this phenomenon without amicroscope we show camera images of a large area sample through apolarizing film in FIG. 4(b) for selected polarizations and voltages.Here, our devices are 1 in² in area and limited by the size and qualityof our existing stamp, however, large area (36 in²) and roll-to-rollnanoimprinting has been demonstrated that could allow scaling of thedevice to hand-held and notebook dimensions with high pattern fidelity.Another critical aspect of the resultant device is color quality. FIG.4(c) shows the CIE chromaticity diagram and microscope images of thesurface as a function of voltage when the polarizer, LC director andanalyzer are parallel. Black dots indicate the coordinates of thereflection spectra and a dotted black line shows the total area thesurface could span if integrated in a color-mixing scheme. Due to theabsorptive nature of the color generating phenomenon, the color gamut isless than that of the NTSC standard, but comparable in area to that ofhigh quality print magazine (AAAS Science).

Angle Dependence of the LC-Plasmonic System

To add to the fundamental study of the LC-plasmonic system, angleresolved reflection measurements using an integrating sphere with arotating center mount were performed (RTC-060-SF, Labsphere Inc.). Aschematic is shown in FIG. 5(a) and depicts the case where incidentlight is polarized parallel to the LC director. Two angles of incidenceare defined: φ, where the polarization of light is parallel to the axisof rotation (TE, s-polarized), and θ, when the polarization of light isperpendicular to the axis of rotation (TM, p-polarized). For theincident angle φ, there is an expected angle independence from theexperimental measurements in FIG. 5(b). However, changes in θ produce ashift in resonant wavelength of the GCSP modes according to

$\frac{\sin}{\lambda} = {{{\pm \frac{1}{\lambda}}\sqrt{\frac{ɛ_{Al}ɛ_{LC}}{ɛ_{LC} + ɛ_{LC}}}} - {\frac{m}{P}.}}$

Here, θ is the angle of incidence, λ is the resonance wavelength, ε_(LC)and ε_(AL) are the dielectric constants of the LC and aluminum,respectively, m is the mode order, and P is the structure periodicity.Black lines depict this GCSP dispersion relation when corrected forrefraction at the top glass and LC interfaces. The results of thesefigures clearly show an area of improvement for a display utilizing thisphenomenon. While an ideal display has little dependence on viewingposition, this device has an angle independence in one axis of rotationbut a dependence in the other. However, by orienting the display alongthe preferred viewing angle, the display can be made almost angleindependent. Furthermore, through the use of a compensation film or ananostructure with alternative plasmonic modes, the viewing angledependence could be improved and optimized.

Active and Passive Addressing Schemes

Once a color gamut is obtained the system can be combined with variousaddressing schemes. A critical advantage of the proposed LC-plasmonicsystem is its facile integration with pre-existing display technologies.To demonstrate this capability, a conventional transmissive TN LCD panelwas used (Adafruit, ID: 1680) and isolated the thin film transistorarray by removing the back light, polarizers, diffusers and ITO glass.The nanostructured aluminum surface is UV cured onto the TFT glass platewith 8.5 μm spacers and then filled with LCM1107. FIG. 6(a) shows amicroscope image of the resulting device (10×) in which the electricalcomponents of the TFT can be clearly seen. Light passes through apolarizer, ITO windows and LC to reflect off the plasmonic surface andback out of the device. The white areas are due to reflection off themetal lines of the TFT. In the voltage-off state, the entire surface iseither blue or red, depending on the polarization of incident light withrespect to the top LC alignment director. By keeping the electronics ofthe TFT intact, the device is interfaced with a computer through aHDMI-to-TTL converter. Individual pixels are then controlled throughimages the computer outputs to the display. FIG. 6(b) shows how thesurface changes color by applying a voltage to every 3^(rd) and 4^(th)row of the TFT. Full images can be displayed as shown in FIG. 6(c).While this shows the ease at which LC-plasmonic systems can beintegrated with existing TFTs, the prototype device has severalengineering challenges to overcome. The white reflection from the TFTmetal lines, meant for use in transmissive displays, tends to wash outcolor from the underlying plasmonic surface. A black matrix needs to besuperimposed on TFT metal lines to mitigate this. Alternatively,fabrication of the plasmonic surface on the TFT glass itself will solvethis unwanted reflection problem. Secondly, the off-the-shelf TFTdrivers only source ˜10 V_(rms); not enough for the surface's transitionto green for the present cell gap of 8.5 μm. TFT drivers capable ofsourcing 15-20 V are needed in conjunction with a reduction in cell gap.The voltage needed for saturation is proportional to the thickness ofthe LC layer, but changes in this gap will alter theintermediate-voltage optical properties of the device. With properengineering of these parameters we believe the max operating voltage ofthe cell can be greatly reduced while maintaining its color changingfunctionality. In absence of the required TFT in present academicsetting, a passively addressed device is shown in FIG. 6(d). Here, UVlithography is performed to macroscopically pattern the nanostructuredsurface, which is followed by a blanket deposition of aluminum. Treatingthis surface as the “common,” we pattern the top ITO glass to haveindividual control of each letter in “UCF.” Furthermore, we utilize a UVphotoalignment material (PAAD-22, Beam Co.) instead of a physicallyrubbed polyimide to demonstrate how the off-state colors can be variedthroughout a single device and with a single laminated polarizer. Thisazobenzene based material, when exposed with linearly polarized UVlight, aligns the LC homogeneously and perpendicularly to thepolarization of exposure. Here, the alignment layer above the “U” isexposed orthogonally with respect to the polarization of exposure abovethe “CF.” The segmented top ITO/photoalignment substrate is then alignedwith the “UCF” plasmonic surface and the resulting cell is infiltratedwith LCM1107. By changing the orientation of the LC alignment directorwith respect to the top polarizer, the “U” and the “CF” change betweenorthogonal colors. Lastly, we apply a field across the “C” to obtain agreen and demonstrate how the three RGB basis colors can be obtainedfrom a single nanostructure/pixel. Lastly, the unoptimized switchingcharacteristics of the device are shown in Supplementary FIG. 5 and showvoltage dependent rise and fall times. Full cycling times are somewhatinvariant to voltage and in the 70 ms range, which equates to ˜14 Hz.While it is believed that this can be improved to standard display framerates, it remains orders of magnitude faster than other reported colorchanging technologies such as electrochemical or translocation basedmethods, which take seconds to tens of seconds to change.

Discussion

In summary, a polarization dependent LC-plasmonic system capable ofproducing the RGB color basis set as a function of voltage and from asingle surface, for the first time, has been produced. This is achievedusing a single, continuous, aluminum nanostructure in conjunction with ahigh birefringence LC. By controlling the deposition conditions of thealuminum, a surface roughness induced polarization dependence isrealized in the presence of a surrounding anisotropic media. Thisphenomena is then exploited through its integration with a LC cell thatperforms as a polarization rotator at low voltages while shifting theplasmonic resonance of the aluminum nanostructure at higher voltages.Large area nanoimprint lithography based scalable fabrication of thenanostructure combined with its ready integration with LC technology canlead to a new class of LC-plasmonic devices.

Methods Fabrication of Nanostructured Surfaces

The nanostructured surface is fabricated through nanoimprint lithography(NIL) for rapid replication. A polymer (dimethylsiloxane) (PDMS; DowCorning, Sylgard) stamp is cast from a master that has been made frombeam lithography (EBL). A thin film of SU-8 2000.5 (MicroChem) was spun(500 rpm for 5 s followed by 3000 rpm for 30 s), then prebaked at 95° C.for 1 min. This film is imprinted with the PDMS stamp (20 s) on thehotplate. The stamp and substrate are removed from the hotplate andallowed to cool (30 s). After stamp delamination, the substrate is UVcured (1 min) and post exposure baked (95° C. for 1 min).

Electron Beam Deposition

The 30 nm Al films are deposited using a Temescal (FC-2000) six-pocketelectron beam evaporation system. For the “smooth” film studied in FIGS.2(c) and (e), the sample is mounted on a thermal electric cooler (TEC)and brought to −20° C. Evaporations are done at pressures of ˜6×10⁻⁶ Tand deposition rates of ˜0.1 nm s⁻¹. For the “rough” film used in FIGS.2(d) and (f) and throughout the rest of this disclosure, the samples areevaporated at room temperature and starting pressures of ˜1×10⁻⁵ T.Before deposition, three edges of the sample were masked off. Thisgreatly reduces the chance of a short circuit in the completed liquidcrystal cell.

LC Cell Formation

The plasmonic LC cell is fabricated using commercially available twistednematic LC cells (AWAT PPW, Poland). The commercial cells are heated to200° C. and then split into two rubbed-polyimide ITO coated glass slideswith 8 μm silica spacers. A single slide is adhered to the plasmonicsurface sample using NOA 81 with the polyimide alignment parallel to thenanostructure grating vector. Once UV cured, the LC-plasmonic cell isinfiltrated with LC (LCM1107, LC Matter Corp.). The LC cells are drivenwith a 1 kHz AC sine wave to reduce ion migration. All reported voltagesare RMS values unless stated otherwise.

Optical Measurements and Images

Reflection spectra are collected using a 4×, 0.07 numerical apertureobjective on an optical microscope (Hyperion 1000) coupled to a Fouriertransform infrared spectrometer (Vertex 80). Reflection spectra arenormalized to an aluminum mirror with 96% reflectivity and a linearpolarizer. Images are collected using the same optical microscope withan Infinity 2-5 camera. Defects due to stamp damage have been replacedby the nearest area in FIG. 5(d) with the GIMP software package.

Finite Difference Time Domain Modeling

Reflection spectra are calculated using experimental parameters for theprinted 2D grating structures, with commercial finite-differencetime-domain (FDTD) software package (Lumerical FDTD, Lumerical SolutionsInc.). The profile for the electromagnetic simulations was obtained byfitting an analytical equation to SEMs of the nanostructured surface(FIG. 2(c-d)). Surface roughness profiles are generated in Matlab andimported into the FDTD simulations. The wavelength dependent refractiveindex of aluminum is taken from Palik and the anisotropic parameters ofthe LC layer are obtained using an effective anisotropic index modelbased on the orientation of LC obtained from FEM calculations.

Watershed-Based Grain Size Determination

Matlab's inbuilt watershed algorithm finds and labels regions surroundedby lines of local minima, or “watershed ridge lines” of arbitrarymatrices. We use this generalized method on SEM images to identifygrains of an aluminum film and their size. In order to effectively usethe function, several image processing steps must be completed with careso that they do not influence the results of the grain sizedetermination.

The method is as follows:

-   1) Define a pixel-to-nm length conversion factor.-   2) Normalize possibly uneven background through Top-hat filtering.-   3) Optional 2D interpolation (will impact pixel to length    conversion).-   4) Gaussian Filter to eliminate static noise (filter must be much    smaller than grain size but larger than static noise of image).-   5) Find complimentary image.-   6) Suppress all image minima less than the noise floor of the image.-   7) Apply watershed algorithm to find local regions surrounded by    lines of local minima and their size.-   8) Superimpose watershed result on image to verify output of    function-   9) Find histogram of watershed results which gives list of local    regions with their respective number of pixels-   10) Convert number of pixels into an area with pixel-to-nm    conversion factor.-   11) As grains are of random shapes, assume grains are spheroidal and    obtain diameter of respective circle given area.-   12) Gaussian fit the resulting histogram plot.    Supplementary FIG. 1 shows the input SEM images for the “rough” and    “smooth” aluminum thin film at various steps in the watershed    method. The method results in a list of regions with the number of    pixels they contain. The area of each region is then converted into    an approximate grain diameter. Regions are mapped back on the image    in the form of white lines to confirm the methods functionality.

FIGS. 8(a-b) shows the off-state behavior of the surface results in blueor red when the electric field of incident light is parallel (FIG. 8(a))or perpendicular (FIG. 8(b)) to the top LC alignment director,respectively. Without a polarizer, the device reflects a superpositionof the two orthogonal polarization colors, resulting in a purple color(FIG. 8(c)).

LC Modeling and Jones Matrix Method

The bulk LC dynamics are simulated using the TechWiz LCD 3D (Sanayi)software package. The Finite Element Method (FEM) solver finds theminimum energy state of the LC director given LC material parameters,boundary conditions and applied voltages. A unit cell of the simulatednano-well array can be seen in FIG. 9(a), where the LC layer isapproximated with 50 layers, which has been found to give well convergedresults. The simulations are run in increments of 0.1 V and result in LCdirector tensors that represent the LC orientation throughout the cell.We then convert this data into the “tilt” (θ) and “twist” (φ) angles ofthe LC, which can be seen in FIG. 9(b). These simulation results willthen be used in a Jones Matrix formulation to find the optical behaviorof the LC cell.

Once the direction of the LC throughout the cell is known, a JonesMatrix formulation is used to solve for the cell's optical properties.This is done by approximating the LC layer as a stack of N uniaxialcrystals and combining their transfer matrices,

$\begin{matrix}{{{LC}_{j} = {\prod\limits_{j = 1}^{N}M_{j}}}{where}} & (1) \\{M_{j} = {{{Rot}\left( {- \phi} \right)}*\begin{bmatrix}e^{{- i}\; \frac{g_{j}}{2\;}} & 0 \\0 & e^{i\; \frac{g_{j}}{2}}\end{bmatrix}{{{Rot}(\phi)}.}}} & (2)\end{matrix}$

Here, Rot is the rotation matrix and φ is the twist angle of the LC fora given layer j.

$\begin{matrix}{{{{Rot}(\phi)} = \begin{bmatrix}{\cos (\phi)} & {\sin (\phi)} \\{- {\sin (\phi)}} & {\cos (\phi)}\end{bmatrix}}{and}} & (3) \\{g_{j} = {\frac{2\pi}{\lambda}\Delta \; n_{j}{d_{j}.}}} & (4)\end{matrix}$

This phase term depends on the thickness, d_(j), and birefringence,Δn_(j), of each individual layer j. The birefringence is given by

Δn _(j) =n _(e)(θ_(j))−n _(o)   (5)

where n_(e)(θ_(j)) is the effective extraordinary index of the LC givena tilt angle of θ_(i)

$\begin{matrix}{\frac{1}{n_{e}^{2}(\theta)} = {\frac{\cos^{2}(\theta)}{n_{e}^{2}} + {\frac{\cos^{2}(\theta)}{n_{o}^{2}}.}}} & (6)\end{matrix}$

Here, n_(e) and n_(o) are the extraordinary and ordinary indices of theliquid crystal.

To find the strength and phase of light at each layer, we iterativelyperform the above matrix multiplication for incident light that isparallel to the top LC alignment director and which we define in thex-direction.

$\begin{matrix}{\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}_{j} = {{LC}_{j}\begin{bmatrix}1 \\0\end{bmatrix}}} & (7)\end{matrix}$

We then use this to find the E-field that excites the plasmonic surface.To determine the amount of each orthogonal mode of the surface that'sexcited, we project the exciting light on the states of the surfacewhich are defined by the in-plane angle of the LC atop it,

$\left. \theta_{N} \right.\sim = {\frac{\pi}{4}.}$α=|

ψ₁|LC_(N) |E _(in)

|  (8)

β=|

ψ₂|LC_(N) |E _(in)

|  (9)

This results in wavelength and voltage dependent weighting factors, αand β, such that the reflection spectra of the surface in the lowvoltage regime, where bulk LC deforms but remains anchored LC onaluminum surface, is a superposition of the two orthogonal off-statemodes.

|ψ_(v)

=α|ψ₁

+β|ψ₂

  (10)

These weighting terms also satisfy the following condition.

α²+β²=1   (11)

The resulting α and β are shown in FIG. 10(a-b), respectively. Here, aneffective flipping occurs within the 3.5 V to 4 V region. Theseweighting values are used on the two orthogonal off-state reflectionspectra of the surface to closely match experimental measured spectra atthis flipping region, as seen in FIG. 3(e). While in FIG. 4(a), theseweighting terms are applied to the FDTD simulated reflection spectra forthe surface's two orthogonal off state modes.

To complete the model and find the reflection out of the device, we mustconsider reflection, a reverse pass through the LC and exit through thepolarizer. It is possible to incorporate these into the matrix method byusing the transfer matrix method in reverse, or by using the following,which is based on the symmetry of the system.

$\begin{matrix}{R = {{\begin{bmatrix}{\cos \; \phi_{1}} & {\sin \; \phi_{1}}\end{bmatrix}{HMH}^{- 1}{M\begin{bmatrix}{\cos \; \phi_{1}} \\{\sin \; \phi_{1}}\end{bmatrix}}}}^{2}} & (12)\end{matrix}$

Where is given by.

$\begin{matrix}{H = \begin{bmatrix}{\cos \; \phi_{N}} & {\sin \; \phi_{N}} \\{\sin \; \phi_{N}} & {{- \cos}\; \phi_{N}}\end{bmatrix}} & (13)\end{matrix}$

FIGS. 11(a-b) shows response time measurements. Using the experimentalsetup outlined in FIG. 3(b) (633 nm He—Ne laser, polarizing beamsplitter and photodiode), measurements of response time are shown for(a) 2.6 V and (b) 5.4 V (1 kHz AC). These give total switching times of70.7 ms and 76.4 ms, respectively. Cell gaps are 8.45 μm.

Actively Tunable Structural Color based on Liquid Crystal -PlasmonicSurfaces (PRV)

FIG. 12 demonstrates actively tunable structural color based on LiquidCrystal—Plasmonic Surfaces (PRV). FIGS. 12(a-h) show dynamic colortuning of arbitrary images. FIGS. 12(a-d) show optical micrographs of asingular Afghan Girl image (National Geographic Society) as a functionof applied electric field. Nanostructure periods are chosen so colorsmatch the original photograph at color tuning saturation, 10 V/μm.Defects due to fabrication errors have been replaced by nearestneighbors. FIG. 12(e) show optical micrograph at 10 V/μm with 10×objective shows pixilation of the image. FIG. 12(f-h) show SEM images ofthe sample before fabrication into a liquid crystal cell. The seriesshows the constituent nanostructure of individual pixels.

The embodied device enables the surface of each pixel to dynamicallychange color. This eliminates the need for subpixels, increasing theresolution of any screen by 66% while using the exact same thin filmtransistor (TFT) array and electronics. This large increase inresolution, without the added cost and difficulty of manufacturingsmaller electronics, give this technology the potential to be trulydisruptive.

All references, including publications, patent applications, and patentscited herein are hereby incorporated by reference in their entireties tothe same extent as if each reference was individually and specificallyindicated to be incorporated by reference and were set forth in itsentirety herein.

The use of the terms “a”, “an”, “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) is to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. The term “connected” is to beconstrued as partly or wholly contained within, attached to, or joinedtogether, even if there is something intervening.

The recitation of ranges of values herein are merely intended to serveas a shorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wasindividually recited herein with an uncertainty unless otherwiseindicated of +/−20 percent.

All methods described herein can be performed in any suitable orderunless otherwise indicated herein or otherwise clearly contradicted bycontext. The use of any and all examples, or exemplary language (e.g.,“such as”) provided herein, is intended merely to better illuminateembodiments of the invention and does not impose a limitation on thescope of the invention unless otherwise claimed.

No language in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. There isno intention to limit the invention to the specific form or formsdisclosed, but on the contrary, the intention is to cover allmodifications, alternative constructions, and equivalents falling withinthe spirit and scope of the invention, as defined in the appendedclaims. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

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What is claimed is:
 1. A liquid crystal-plasmonic display apparatus,comprising: a linear polarizer; a superstrate; a transparent electrode;a rubbed polyimide film; a liquid crystal (LC); and a nano-structuredplasmonic surface disposed on a substrate, where the transparentelectrode is disposed to apply an electric field across the liquidcrystal, and the rubbed polyimide aligns the LC parallel to the axis itis rubbed to provide homogeneous alignment, wherein the LC orientationnear the plasmonic surface determines the effective refractive index ofpolarization-sensitive, grating-coupled surface plasmon (GCSP) modesand, therefore, the resonant wavelength of the nano-structured plasmonicsurface, further wherein the apparatus is characterized by a reflectivecolor changing surface capable of producing the full RGB color basisset, all as a function only of voltage and based on a singlenanostructure.
 2. The apparatus of claim 1, wherein the nano-structuredplasmonic surface is a rough surface, inducing polarization dependenceon the GSCP resonance in the presence of an anistropic media.
 3. Theapparatus of claim 1, wherein the nano-structured plasmonic surface is asmooth surface, resulting in substantial polarization independence. 4.The apparatus of claim 1, wherein the LC is a cell having at least aportion thereof comprised of the nano-structured plasmonic surface. 5.The apparatus of claim 1, wherein the nano-structured plasmonic surfaceis a nano-structured aluminum surface.
 6. The apparatus of claim 1,wherein the superstrate is a glass composition.
 7. The apparatus ofclaim 1, wherein the transparent electrode comprises indium tin oxide(ITO).
 8. The apparatus of claim 1, further comprising a cell gap of theLC measuring approximately 8.5 μm.
 9. A method of displaying a tunablecolor image, comprising: providing the liquid crystal-plasmonic displayapparatus of (1); transmitting incident unpolarized ambient lightthrough the linear polarizer, the glass superstrate, the transparentelectrode, and the rubbed polyimide film, wherein the incidentunpolarized light becomes polarized, passes through the highbirefringence LC layer, and excites grating coupled surface plasmons(GSCP) on the nanostructured surface; applying a voltage across theplasmonic film and the top electrode, thereby controlling theorientation of the LC throughout the cell; and varying the voltagebetween a low and a high range.
 10. The method of claim 9, furthercomprising the step of adjusting the angle of the linear polarizer inrelation to the LC.
 11. The method of claim 9, further comprising thestep of aligning the linear polarizer parallel to the LC.
 12. The methodof claim 11, further comprising the step of reflecting a blue color fromthe liquid crystal-plasmonic display apparatus.
 13. The method of claim9, further comprising the step of aligning the linear polarizerperpendicular to the LC.
 14. The method of claim 13, further comprisingthe step of reflecting red color from the liquid crystal-plasmonicdisplay apparatus.
 15. The method of claim 9, wherein thenano-structured plasmonic surface is a nano-structured aluminum surface.16. The method of claim 9, wherein the superstrate is a glasscomposition.
 17. The method of claim 9, wherein the transparentelectrode comprises indium tin oxide (ITO).